Strange behavior when manipulating char* array in C - c

Question

I'm trying to create a command line shell for an Operating Systems class. One of our assignments is to create a builtin "history" command that prints out the last 10 commands executed in the shell. Here is the code I have written for the "history" command:
char* cmd_hsitory[10]; // This is a global variable
int add_history(char **args) {
cmd_history[9] = NULL;
for(int i = 8; i >= 0; i--) {
cmd_history[i+1] = cmd_history[i];
}
cmd_history[0] = *args;
return 1;
}
Where the char **args argument is the last command exectued. Here is the function that prints the history:
int lsh_history(char **args) {
printf("Last 10 commands: \n");
for(int i = 0; i < 10; i++) {
printf("%s\n", cmd_history[i]);
}
return 1;
}
Some strange behavior is happening with this code. For instance, when I run the commands [cd, cd, ls, history] in succession, this is the printed output:
Last 10 commands:
ls
ls
cd
(null)
(null)
(null)
(null)
(null)
(null)
(null)
The first problem here is that I ran the cd command twice, and the ls command only once. If I run the "history" command again I get:
Last 10 commands:
history
ls
ls
cd
(null)
(null)
(null)
(null)
(null)
(null)
This seems correct with the exception of the 2 ls commands vs the 1 cd command.
This isn't very consistent, though, as sometimes I'll get mixed-up commands and the "history" command will show up several times.
If someone told me what's glaringly wrong with my code, that would be of great help. Thanks!
EDIT: Here is the full source code:
P.s. Most of this code is pulled from the internet (Stephen Brennan) and I'm building on top of it to learn. I will not submit this code as my assignment.
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/*
Function Declaration for history queue
*/
int add_history(char **args);
/*
Function Declarations for builtin shell commands:
*/
int lsh_cd(char **args);
int lsh_help(char **args);
int lsh_exit(char **args);
int lsh_history(char **args);
/*
List of builtin commands, followed by their corresponding functions.
*/
char *builtin_str[] = {
"cd",
"help",
"exit",
"history"
};
int (*builtin_func[]) (char **) = {
&lsh_cd,
&lsh_help,
&lsh_exit,
&lsh_history
};
char *cmd_history[10];
int lsh_num_builtins() {
return sizeof(builtin_str) / sizeof(char *);
}
int add_history(char **args) {
cmd_history[9] = NULL;
for(int i = 8; i >= 0; --i) {
cmd_history[i+1] = cmd_history[i];
}
cmd_history[0] = NULL;
cmd_history[0] = *args;
return 1;
}
/*
Builtin function implementations.
*/
/**
#brief Builtin command: command history.
#param args List of args. args[0] is "history".
#return Always returns 1 to continue executing.
*/
int lsh_history(char **args) {
printf("Last 10 commands: \n");
for(int i = 0; i < 10; i++) {
printf("%s\n", cmd_history[i]);
}
return 1;
}
/**
#brief Bultin command: change directory.
#param args List of args. args[0] is "cd". args[1] is the directory.
#return Always returns 1, to continue executing.
*/
int lsh_cd(char **args)
{
if (args[1] == NULL) {
chdir("/Users/Landon/");
} else {
if (chdir(args[1]) != 0) {
perror("lsh");
}
}
return 1;
}
/**
#brief Builtin command: print help.
#param args List of args. Not examined.
#return Always returns 1, to continue executing.
*/
int lsh_help(char **args)
{
int i;
printf("Stephen Brennan's LSH\n");
printf("Type program names and arguments, and hit enter.\n");
printf("The following are built in:\n");
for (i = 0; i < lsh_num_builtins(); i++) {
printf(" %s\n", builtin_str[i]);
}
printf("Use the man command for information on other programs.\n");
return 1;
}
/**
#brief Builtin command: exit.
#param args List of args. Not examined.
#return Always returns 0, to terminate execution.
*/
int lsh_exit(char **args)
{
return 0;
}
/**
#brief Launch a program and wait for it to terminate.
#param args Null terminated list of arguments (including program).
#return Always returns 1, to continue execution.
*/
int lsh_launch(char **args)
{
pid_t pid;
int status;
pid = fork();
if (pid == 0) {
// Child process
if (execvp(args[0], args) == -1) {
perror("lsh");
}
exit(EXIT_FAILURE);
} else if (pid < 0) {
// Error forking
perror("lsh");
} else {
// Parent process
do {
waitpid(pid, &status, WUNTRACED);
} while (!WIFEXITED(status) && !WIFSIGNALED(status));
}
return 1;
}
/**
#brief Execute shell built-in or launch program.
#param args Null terminated list of arguments.
#return 1 if the shell should continue running, 0 if it should terminate
*/
int lsh_execute(char **args)
{
int i;
if (args[0] == NULL) {
// An empty command was entered.
return 1;
}
for (i = 0; i < lsh_num_builtins(); i++) {
if (strcmp(args[0], builtin_str[i]) == 0) {
return (*builtin_func[i])(args);
}
}
return lsh_launch(args);
}
#define LSH_RL_BUFSIZE 1024
/**
#brief Read a line of input from stdin.
#return The line from stdin.
*/
char *lsh_read_line(void)
{
int bufsize = LSH_RL_BUFSIZE;
int position = 0;
char *buffer = malloc(sizeof(char) * bufsize);
int c;
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
while (1) {
// Read a character
c = getchar();
if (c == EOF) {
exit(EXIT_SUCCESS);
} else if (c == '\n') {
buffer[position] = '\0';
return buffer;
} else {
buffer[position] = c;
}
position++;
// If we have exceeded the buffer, reallocate.
if (position >= bufsize) {
bufsize += LSH_RL_BUFSIZE;
buffer = realloc(buffer, bufsize);
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
}
}
#define LSH_TOK_BUFSIZE 64
#define LSH_TOK_DELIM " \t\r\n\a"
/**
#brief Split a line into tokens (very naively).
#param line The line.
#return Null-terminated array of tokens.
*/
char **lsh_split_line(char *line)
{
int bufsize = LSH_TOK_BUFSIZE, position = 0;
char **tokens = malloc(bufsize * sizeof(char*));
char *token, **tokens_backup;
if (!tokens) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
token = strtok(line, LSH_TOK_DELIM);
while (token != NULL) {
tokens[position] = token;
position++;
if (position >= bufsize) {
bufsize += LSH_TOK_BUFSIZE;
tokens_backup = tokens;
tokens = realloc(tokens, bufsize * sizeof(char*));
if (!tokens) {
free(tokens_backup);
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
token = strtok(NULL, LSH_TOK_DELIM);
}
tokens[position] = NULL;
return tokens;
}
/**
#brief Loop getting input and executing it.
*/
void lsh_loop(void)
{
char *line;
char **args;
int status;
do {
printf("> ");
line = lsh_read_line();
args = lsh_split_line(line);
status = lsh_execute(args);
add_history(args);
free(line);
free(args);
} while (status);
}
/**
#brief Main entry point.
#param argc Argument count.
#param argv Argument vector.
#return status code
*/
int main(int argc, char **argv)
{
// Load config files, if any.
// Run command loop.
lsh_loop();
// Perform any shutdown/cleanup.
return EXIT_SUCCESS;
}

Answer 1

You need to strdup() the arg strings over to your cmd_history pointers, not just have your cmd_history pointers reference the original command string. When you free(line), you're putting the memory that your cmd_history pointers are referencing back into the free pool. In the next iteration of the loop you may or may not overwrite that data.

Answer 2

You wrote ((Comments Added)):
void lsh_loop(void)
{
...
line = lsh_read_line(); //mallocs line
args = lsh_split_line(line); //mallocs args array; consisting of references into line[]
status = lsh_execute(args); //no promblem here
add_history(args); // ***PERMANETNLY*** stores args[0] into cmd_history[]!!!
free(line); //frees line[]
free(args); //frees the pointer array into freed line[], which was copied to cmd_history[] --> cmd_history[] points to unallocated memory!!!
}
The main problem is related to strtok(): It returns a pointer into the spitted string, within the original string: If you later free the token pointer array or the original string and use it later --> UB.
Additionally lsh_history() kills also last entry which would leave a memory leak behind, if you would allocated that entry firmly.
Solution:
Establish for cmd_history[] your own storage of the strings, duplicate and free them properly in lsh_history().